In nature, the organic constituents of animals, microorganisms and plants are made up of chiral molecules, or molecules which exhibit handedness. Enantiomers are stereoisomers or chiral molecules whose configurations (arrangements of constituent atoms) are nonsuperimposed mirror images of each other; absolute configurations at chiral centers are determined by a set of rules by which a priority is assigned to each substituent and are designated R and S. The physical properties of enantiomers are identical, except for the direction in which they rotate the plane of polarized light: one enantiomer rotates plane-polarized light to the right and the other enantiomer rotates it to the left. However, the magnitude of the rotation caused by each is the same.
The chemical properties of enantiomers are also identical, with the exception of their interactions with optically active reagents. Optically active reagents interact with enantiomers at different rates, resulting in reaction rates which may vary greatly and, in some cases, at such different rates that reaction with one enantiomer or isomer does not occur. This is particularly evident in biological systems, in which stereochemical specificity is the rule because enzymes (biological catalysts) and most of the substrates on which they act are optically active.
A mixture which includes equal quantities of both enantiomers is a racemate (or racemic modification). A racemate is optically inactive, as a result of the fact that the rotation of polarized light caused by a molecule of one isomer is equal to and in the opposite direction from the rotation caused by a molecule of its enantiomer. Racemates, not optically active compounds, are the products of most synthetic procedures. Because of the identity of most physical characteristics of enantiomers, they cannot be separated by such commonly used methods as fractional distillation (because they have identical boiling points), fractional crystallization (because they are equally soluble in a solvent, unless it is optically active) and chromatography (because they are held equally tightly on a given adsorbent, unless it is optically active). As a result, resolution of a racemic mixture into enantiomers is not easily accomplished and can be costly and time consuming.
Recently, there has been growing interest in the synthesis of chiral compounds because of the growing demand for complex organic molecules of high optical purity, such as insect hormones and pheromones, prostaglandins, antitumor compounds, and other drugs. This is a particularly critical consideration, for example, for drugs, because in living systems, it often happens that one enantiomer functions effectively and the other enantiomer has no biological activity and/or interferes with the biological function of the first enantiomer.
In nature, the enzyme catalyst involved in a given chemical reaction ensures that the reaction proceeds asymmetrically, producing only the correct enantiomer (i.e., the enantiomer which is biologically or physiologically functional). This is not the case in laboratory synthesis, however, and, despite the interest in and energy expended in developing methods by which asymmetric production of a desired chiral molecule (e.g., of a selected enantiomer) can be carried out, there has been only limited success.
In addition to resolving the desired molecule from a racemate of the two enantiomers, it is possible, for example, to produce selected asymmetric molecules by the chiral pool or template method, in which the selected asymmetric molecule is "built" from pre-existing, naturally-occurring asymmetric molecules. Asymmetric homogeneous hydrogenation and asymmetric epoxidation have also been used to produce chiral molecules. Asymmetric hydrogenation is seen as the first manmade reaction to mimic naturally-occurring asymmetric reactions. Sharpless, K. B., Chemistry in Britain, Jan. 1986, pp 38-44; Mosher, H. S. and J. D. Morrison, Science, 221:1013-1019 (1983); Maugh, T. H., Science, 221:351-354 (1983); Stinson, S., Chemistry and Engineering News, 24, Jun. 2, 1986.
Presently-available methods of asymmetric synthesis are limited in their applicability, however. Efficient catalytic asymmetric synthesis reactions are very rare; and they usually require a directing group and thus are substrate limited. Because such reactions are rare and chirality can be exceptionally important in drugs, pheromones and other biologically functional compositions, a catalytic method of asymmetric dihydroxylation would be very valuable. In addition, many naturally-occurring products are dihydroxylated or can be easily derived from a corresponding vicinal diol derivative.